1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Rotating field sensors are used not only to detect the rotational position of an object but also to detect a linear displacement of an object. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in conjunction with the rotation or linear movement of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position or linear displacement of the object is thus detected.
There has been known a rotating field sensor that has two bridge circuits (Wheatstone bridge circuits) as shown in U.S. Pat. Nos. 6,943,544 B2, 6,633,462 B2, and U.S. Patent Application Publication No. 2009/0206827 A1. In such a rotating field sensor, each of the two bridge circuits includes four magnetoresistive elements (hereinafter referred to as MR elements) serving as magnetic detection elements. Each of the bridge circuits detects the intensity of a component of the rotating magnetic field in one direction, and outputs a signal that indicates the intensity. The output signals of the two bridge circuits differ in phase by ¼ the period of the output signals of the bridge circuits. The angle that the direction of the rotating magnetic field forms with respect to a reference direction is calculated based on the output signals of the two bridge circuits.
In a rotating field sensor that uses MR elements as the magnetic detection elements, the waveforms of the output signals of the MR elements corresponding to the resistance values ideally trace a sinusoidal curve (including a sine waveform and a cosine waveform) as the direction of the rotating magnetic field rotates. However, it is known that the waveforms of the output signals of the MR elements can be distorted from a sinusoidal curve, as described in U.S. Pat. No. 6,633,462 B2. If the waveforms of the output signals of the MR elements are distorted, the angle detected by the rotating field sensor may include some error. One of the causes of the distortion of the output signal waveforms of the MR elements is the MR elements themselves.
A description will now be given of an example in which the output signal waveform of an MR element is distorted due to the MR element itself. Here, assume that the MR element is a giant magnetoresistive (GMR) element or a tunneling magnetoresistive (TMR) element. The GMR or TMR element includes a magnetization pinned layer whose direction of magnetization is pinned, a free layer whose direction of magnetization varies according to the direction of the rotating magnetic field, and a nonmagnetic layer disposed between the magnetization pinned layer and the free layer. Examples of the situation where the output signal waveform of an MR element is distorted due to the MR element itself include the case where the free layer has an induced magnetic anisotropy. The induced magnetic anisotropy of the free layer occurs, for example, when the rotating field sensor is installed at a predetermined location and thereafter the location of installation of the rotating field sensor including the MR element once increases and then decreases in temperature with an external magnetic field kept applied to the MR element in a specific direction. If the free layer has an induced magnetic anisotropy, the direction of magnetization of the free layer cannot accurately follow the direction of the rotating magnetic field. As a result, the output signal waveform of the MR element is distorted from a sinusoidal curve.
U.S. Pat. No. 6,633,462 B2 discloses a magnetoresistive sensor including a main sensing element having a main reference magnetization axis, and two correction sensing elements having their respective reference magnetization axes inclined with respect to the main reference magnetization axis. The two correction sensing elements are electrically connected to the main sensing element to correct the detected angle. The technology disclosed in U.S. Pat. No. 6,633,462 B2 is useful for reducing an error signal included in the output signal of the main sensing element if any relationship between the phase of the error signal and the phase of an ideal output signal of the main sensing element is known in the design stage of the sensor.
However, if there occurs an induced magnetic anisotropy of the free layer after the installation of the rotating field sensor as described above, the induced magnetic anisotropy causes the direction of easy magnetization to be oriented in an arbitrary direction. Therefore, in this case, the relationship between the phase of an error component included in an output signal of an MR element and the phase of an ideal output signal of the MR element is not constant, and thus cannot be known in the design stage of the rotating field sensor. For this reason, the technology disclosed in U.S. Pat. No. 6,633,462 B2 is not applicable to the case where there occurs an induced magnetic anisotropy of the free layer after the installation of the rotating field sensor.
The foregoing descriptions have dealt with a rotating field sensor that employs MR elements as magnetic detection elements, especially focusing on the problem of an error component having an arbitrary phase due to the occurrence of an induced magnetic anisotropy of the free layer after the installation of the rotating field sensor. However, this problem holds true for all cases where, in a rotating field sensor that includes at least one magnetic detection element and detects an angle that the direction of a rotating magnetic field forms with respect to a reference direction, an error component included in the output signal of the magnetic detection element has an arbitrary phase.